Nginx C Module Debug

nginx.org C Module Debugging Best Practices

Comprehensive debugging guide for nginx C modules, derived from the official nginx development documentation and production debugging experience. Contains 45 rules across 8 categories, prioritized by impact to guide systematic diagnosis of crashes, memory bugs, and behavioral issues in nginx modules.

Companion skills: This skill complements nginx-c-modules (correctness) and nginx-c-module-perf-reliability (performance). This skill covers debugging and diagnosis.

When to Apply

Reference these guidelines when:

• Diagnosing nginx worker crashes (segfaults, SIGABRT, SIGSEGV)
• Finding memory bugs (use-after-free, leaks, pool corruption, buffer overruns)
• Setting up GDB and core dump analysis for nginx
• Tracing request flow through phases, subrequests, and filter chains
• Instrumenting nginx modules with debug logging and dynamic tracing tools

Rule Categories by Priority

Quick Reference

1. Crash Diagnosis & Signals (CRITICAL)

• crash-segfault-signature - Identify Segfault Crash Signature from Signal and Address
• crash-null-deref-pattern - Recognize NULL Pointer Dereference Patterns in nginx Modules
• crash-double-free-finalize - Diagnose Double Finalize Crashes from Request Reference Count
• crash-stack-overflow - Detect Stack Overflow from Recursive Subrequest or Filter Chains
• crash-worker-exit-log - Extract Crash Context from Worker Exit Log Messages
• crash-error-page-redirect - Avoid Crashes from error_page Internal Redirect Context Invalidation

2. Memory Bug Detection (CRITICAL)

• memdbg-use-after-free - Detect Use-After-Free from Pool Destruction Timing
• memdbg-pool-leak-pattern - Identify Pool Memory Leak Patterns from Growing Worker RSS
• memdbg-slab-corruption - Diagnose Shared Memory Slab Corruption from Multi-Worker Crashes
• memdbg-cleanup-handler-leak - Detect Resource Leaks from Missing Pool Cleanup Handlers
• memdbg-buffer-overrun - Find Buffer Overrun from ngx_pnalloc Size Miscalculation
• memdbg-temp-pool-misuse - Avoid Storing Long-Lived Pointers in Temporary Pools
• memdbg-valgrind-pool-trace - Use Valgrind Pool-Level Tracing to Find Leaked Allocations

3. GDB & Core Dump Analysis (HIGH)

• gdb-coredump-setup - Configure Core Dump Generation for nginx Worker Crashes
• gdb-attach-worker - Attach GDB to a Running nginx Worker Process
• gdb-backtrace-read - Read nginx Backtrace to Identify Crash Module and Phase
• gdb-inspect-request - Inspect ngx_http_request_t Fields in GDB for Request State
• gdb-memory-buffer-extract - Extract Debug Log from Memory Buffer Using GDB Script
• gdb-watchpoint-corruption - Use GDB Watchpoints to Catch Memory Corruption at Write Time

4. Request Flow Tracing (HIGH)

• trace-phase-handler-flow - Trace Request Through HTTP Phase Handlers
• trace-subrequest-tree - Map Subrequest Parent-Child Relationships for Debugging
• trace-filter-chain-order - Trace Filter Chain Execution Order and Data Flow
• trace-upstream-callback-seq - Trace Upstream Callback Sequence for Proxy Debugging
• trace-event-handler-chain - Trace Event Handler Execution for Connection Debugging
• trace-config-inheritance - Trace Configuration Inheritance Through Server and Location Blocks

5. Debug Logging Patterns (MEDIUM-HIGH)

• dbglog-debug-mask - Use Correct Debug Mask for Targeted Log Filtering
• dbglog-debug-connection - Use debug_connection to Isolate Single-Client Debug Output
• dbglog-memory-buffer - Use Memory Buffer Logging to Capture Debug Output Without Disk I/O
• dbglog-log-action-string - Set Log Action String for Context in Error Messages
• dbglog-format-ngx-str - Format ngx_str_t Correctly in Debug Log Messages

6. State & Lifecycle Debugging (MEDIUM)

• state-connection-lifecycle - Track Connection State Transitions for Lifecycle Debugging
• state-upstream-state-machine - Debug Upstream Module State by Logging Transition Points
• state-timer-leak - Detect Timer Leaks from Events Not Removed Before Pool Destruction
• state-event-flag-debug - Inspect Event Flags to Debug Unexpected Handler Invocation
• state-request-count-track - Track Request Reference Count to Debug Premature Destruction

7. Dynamic Tracing Tools (MEDIUM)

• probe-strace-syscall - Use strace to Trace System Call Patterns in nginx Workers
• probe-dtrace-request - Trace Request Processing with DTrace pid Provider
• probe-systemtap-pool - Trace Memory Pool Allocations with SystemTap
• probe-ebpf-latency - Measure Per-Function Latency with eBPF Probes
• probe-strace-fd-leak - Detect File Descriptor Leaks with strace and /proc

8. Build & Sanitizer Configuration (LOW-MEDIUM)

• build-debug-flags - Compile nginx with Full Debug Symbols and No Optimization
• build-asan-configure - Build nginx with AddressSanitizer for Memory Error Detection
• build-single-process - Use Single-Process Mode for Simplified Debugging
• build-valgrind-suppressions - Use nginx Valgrind Suppressions to Reduce False Positives
• build-debug-palloc - Enable NGX_DEBUG_PALLOC for Fine-Grained Pool Allocation Tracking

How to Use

Read individual reference files for detailed explanations and code examples:

• Section definitions - Category structure and impact levels
• Rule template - Template for adding new rules

Reference Files